How to separate hollow cylinder into rectangular cubes
You can achieve a more flexible result by using array modeifier. You can easily change all the pieces by editing one piece and changing the gap between pieces as you like.

First apply the solidify modifier and the create a piece of cube by deleting the rest as



After doing this add faces on two sides of the mesh by selecting edges around and pressing F



Now you have one piece of rectangle. Now add an empty -> Plain axes in the origin of the mesh. This empty will be used to rotate the array around the centre to create a perfect circle. So add it in the origin of the mesh that we have now.



Now add an array modifier in the mesh and uncheck object offset and check on relative offset and select the empty we created in object offset field. this will let us use empty we just created to control the mesh. 




Now we can rotate the empty on Z axis to create as much gapping as we want between the pieces. You can play with the array count to create a cylinder like mesh with the rectangles.enter image description here

You can apply the array modifier and change individual pieces if that is what you want. I hope this helps.